Well, the thing about dreams in Inception is that they aren't normal dreams. They're carefully constructed mazes built to serve a very specific purpose, which is to fool the sleeper and allow the extractor to obtain some piece of information hiding in the sleeper's mind. If the dreams constantly shifted and strange things happened, then it would more easily clue the sleeper into the fact that it's all a dream. BUT it would also make the extractor's job that much harder, because they're constantly having to deal with changes in location, characters, rules, and so on. Essentially, the whole thing would be rather useless.